I know that kvm-qemu supports nested virtualization for Intel (VT-x) on Linux, since I have this working. I don't have this working on my mac with os x as a host though.
My Macbook Air with Dual-Core Intel i5 supports VT-x virtualization. I know this because this command shows 'VMX':
$ sysctl -a | grep machdep.cpu.features
I start qemu (qemu-system-x86_64, version 4.2.0) with this command:
$ qemu-system-x86_64 -m 2048 -vga virtio -usb -device usb-tablet -show-cursor -enable-kvm -drive file=~/vms-qemu/lmde.qcow2 -accel hvf -cpu host,vmx
This gives me a warning: 'host doesn't support requested feature: CPUID.01H:ECX.vmx [bit 5]'.
Within the virtual machine, the guest running Linux Mint Debian Edition, virtualization is not available, as expected given the warning. This is shown by the command virt-host-validate from the libvirt-clients package. Also the command lscpu does not show:
Hypervisor vendor: KVM
Virtualization type: full
Why does Qemu warn that the host does not support VMX? I ask because the host /does/ support this feature. And it's enabled by default by Apple since many years (https://support.apple.com/en-us/HT203296).
I successfully enabled nested virtualization on a Linux host with a Linux guest in qemu (2.8.1). For this I had to set the option 'options kvm-intel nested=Y' in /etc/modprobe.d/kvm-intel.conf. Is there perhaps something like this required on Mac OSX to make VT-x hardware assisted virtualization available for the guest?